1. Field of the Invention
The present invention relates to a drive controlling apparatus for a movable member, and more particularly, it relates to a drive controlling apparatus for a movable member for which a range of movement is limited due to mechanical limitations and the like.
2. Related Background Art
In a normal driving system wherein a rotational force generated by a motor is transmitted to a load through a gear transmission mechanism, when a range of movement of a movable member constituting the load is mechanically limited, it is necessary to quickly detect the fact that the movable member reaches the limitation (in other words, the movable member is abutted against a stop and the like for limiting the range of the movement of the movable member) to prevent damage of the motor and/or stops.
FIG. 8 schematically shows a conventional driving mechanism wherein a movable member is shifted linearly. A rotational movement of a gear 1 fixed to a motor shaft 1A is converted into a longitudinal linear movement of a rack 2 constituting the movable member. The movement of the rack is limited by abutting an edge 2a or 2b of the rack 2 against a stop member 3 or 4.
In FIG. 8, if, after the rack 2 is abutted against the stop member 3 or 4, the rack continues to be forced against the stop member, the stop member 3 or 4 and/or the driving motor may be damaged. Normally, when such driving system is servo-controlled, a control circuit (not shown) always monitors the moving of the movable member, and emits a command signal for stopping the driving system it when detects the fact that the movable member is not moved.
The principle thereof is as shown is FIG. 9. FIG. 9 shows an example of a method for detecting the fact that the movable member reaches one of the limitations (stops). In FIG. 9, (a) designates a pulse signal generated according to the movement of the movable member, (b) designates an output from a timer in a detection circuit (not shown). The timer is a forwardly counting timer wherein the counting value representative of the output obtained by counting reference clocks from a oscillation circuit is increased.
Each time when the pulse (a) is inputted, the counting value (b) outputted from the timer is reset to zero. If the pulse (a) is not inputted within a predetermined time te after the movable member 2 is stopped by abutting against the stop 3 or 4, the counting value in the timer reaches a comparison value h corresponding to the predetermined time te, whereby a signal (c) is generated from the timer, thus detecting the fact that the movable member reaches the stop and is stopped there (hereinafter, the above-mentioned predetermined time is referred to as "time-out time period", and the detection at the limitation is referred to as "time-out").
That is to say, the comparison value h corresponds to the time-out time period te for detecting the pulse output when the movable member is not moved, due to reaching the limitation. And, if a the pulse is outputted in correspondence to the movement of the movable member within this time period te, the counting value in the timer is cleared to zero each time when such pulse is outputted, with the result that the time-out signal (c) is not outputted. However, if a pulse is not outputted within such time period, the time-out signal (c) is outputted, whereby the control circuit (not shown) detects the fact that the movable member reaches the limitation, thus stopping the supply of current to the motor.
For example, in an auto-focus control system for performing an focusing operation by rotating an optical system holding cylinder of a taking lens in a camera having an auto-focusing device, the taking lens is normally servo-controlled so that the taking lens is focused to an object, if measurement of distance is correctly performed. However, for an object having low contrast and/or low brightness, or an object having a particular pattern, for example, such as fine continuous patterns, since the ability of measurement of distance is inadequate or insufficient, there will be an error in the measurement of distance. In such a case, the taking lens is further moved from its infinity position or nearest taking position.
Further, if the taking lens is desired to be focused to the object situated ahead of the nearest taking position, the driving mechanism tries to further move the taking lens beyond the nearest taking position However, normally, in such camera, since mechanical stops are arranged in the infinity position and the nearest taking position, the taking lens cannot be moved beyond such stops. Accordingly, in the above-mentioned case, the movable member (taking lens) will be abutted against one of the stops.
The servo-control system in the auto-focus apparatus mentioned above is a typical example of a driving system wherein a rotational force is transmitted to a load through a gear transmission mechanism and the range of movement of the load (movable member) is mechanically limited, as mentioned above. In the driving operation in such auto-focus apparatus, the movable member will also abut against a stop, in the case where, a focusing condition detecting means determines that the measurement of distance cannot be effected. In such case, whether the measurement of distance can be performed in any position or positions within a photo-possible range may be determined by forcibly moving the taking lens through such photo-possible range (i.e., in a so-called scanning mode).
In any case, when the movable member reaches the stop, since the driving system is forcibly stopped, a pulse corresponding to the movement of the movable member will not be outputted. Accordingly, the driving system is stopped when the fact that such pulse is not outputted is detected.
In practice, the rotation speed of the motor varies in response to the fluctuation of the load or the supply voltage, and, in particular, when the motor is started or when the motor is reversed after the motor is once stopped, it takes some time until the motor reaches its steady rotation speed after the motor is started, due to the starting characteristic of the motor. Accordingly, it is difficult to set the time-out time period for detection of a pulse to a short time duration uniformly. Thus, in consideration of the above problem, the time-out time period must be long enough not to detect an erroneous limitation.
However, if the time-out time period is set to be long, when the load (movable member) really reaches the limitation (stop), it is impossible to stop the supply of current to the motor quickly. For this reason, in the conventional driving system, the motor is subjected to large mechanical stress, causing vibration noise, and causing mechanical damage or bad mechanical operation, as the case may be.